lavfi/pixdesctest: fix chroma subsampling with odd sizes.
lavfi/mptestsrc: fix chroma subsampling with odd sizes.

This makes no difference in practice since w & h are hardcoded to 512.
jpeg2000: make sure s->numXtiles / s->numYtiles are reset when tile is deallocated

This keeps the structure fields more consistent after cleanup

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>